A fluid loss additive is an important treatment agent for drilling fluid in oil drilling operation. It reduces the filter loss of a drilling fluid as far as possible and reduces the harmful fluid loss to the stratum by forming a flexible thin and compact filter cake with low permeability on the wall of a well, so as to ensure the drilling work going on wheels.
With the increase of the demand for energy in the world and the development of drilling technology, the stratums being drilled become more and more complicated. The numbers of special wells, ultra-deep wells and complex wells are increasing; therefore, there exists increasing high demands for improved drilling fluid technology. For example, when the temperature of the bottom hole of a deep well rises above 200° C., it has notable effect on the properties of water-based drilling fluid. Therefore, the main problem of the water-based drilling fluid of deep well is high temperature resistance, and the key is the high temperature resisting treatment agent.
A polymer fluid loss additive is one of the most critical treatment agents that ensure the performance of the drilling fluid. Currently, the amount of polymer fluid loss additives used in the application has to be increased in order to play the role of reducing the filter loss in brine drilling fluid and at high temperature conditions. However, as the amount of polymer additives increases, the viscosity of the drilling fluid also increases due to their high relative molecular weight, thus bringing trouble to the application and limiting the use of the polymer treatment agent. Existing polymer fluid loss additives do not possess good high temperature-resistance properties, which causes a great change of rheological properties of the drilling fluid system at high temperature conditions. Thickening and/or degradation of the drilling fluid occurs at high temperature, leading to poor performance of drilling operation. For example, fluid loss additives such as sulfonated phenolic resin (SMP) meet the requirements for drilling within 200° C. However, they can't meet the requirements for high temperature and high pressure filter loss control and requirements for the control of rheology of brine and saturated brine drilling fluid under ultra-high temperature conditions.
Other high temperature-resisting fluid loss additives applicable for different stratigraphic conditions were developed in the prior art, and used in the field application. For example, polymer treatment agents with different components and different relative molecular weights synthesized by copolymerizing monomers of acryloyloxybutyl sulfonic acid, sodium 2-acryloxy-2-vinylmethyl propylsulfonate and N,N-dimethyl acryamide with acrylamide and acrylic acid and the like are disclosed by Zhonghua Wang, et al, in “Studies on Ultra-High Temperature Drilling Fluid System (II)—Synthesis and Evaluation of Polymer Fluid Loss Additives,” PETROLEUM DRILLING TECHNIQUES, 2009(7), having good thermal stability and fluid loss effect in both fresh water and salt water drilling fluids. A novel high temperature water-based fluid loss additive CMJ-1 for drilling fluid was also disclosed, by Jinsheng Sun, et al, in “Development of CMJ-1 High Temperature Film-forming Fluid Loss Additive and Properties thereof,” PETROLEUM DRILLING TECHNIQUES, 2004.3, which is synthesized with ethylene, acetic acid, methanol and sulfonating agent as main starting materials by high temperature oxidation, esterification, polymerization, alcoholysis and sulfonation, and it can form compact isolated films with temperature resistant up to 180° C.
U.S. Pat. No. 5,789,349 discloses an improved water-based drilling fluid system containing a controlled cross-linking polymeric fluid loss additive with good thermal stability, wherein the controlled cross-linking polymer is a copolymerization reaction product of a mixture of acrylamide, monomers with sulfonic groups, non-sulfonated anionic monomers and cross-linking agents with a certain concentration, which has temperature resistance up to 200° C.
U.S. Pat. No. 6,465,587 discloses a copolymer capable of controlling the properties of drilling fluid, which is prepared from the copolymerization reaction of acrylamide, 3-allyloxyhydroxypropanesulfonate (AHPS) and other monomers. The AHPS component is thermally and hydrolyticly stable at high pH and in saturated salt conditions, and the copolymers are especially efficacious as oil well cement slurry additives for rheology and fluid loss control at high temperature.
U.S. Pat. No. 7,651,980 discloses a water-based drilling fluid system mainly consisted of water soluble polymer comprising acrylamide monomer which can control filtration at high temperature. The system is consisted of freshwater, plugging agent with concentration of (21-58) kg/m3, and an amount of bridging agent which is consisted of water-soluble polymer, and the system have good rheological properties. When contacting with a stratum with monovalent salt concentrations of no more than 10,000 ppm, the drilling fluid has effective fluid loss control, and after static aging for a period of 16 hours, with a HTHP (149° C.) filtrate amount of not more than 25 ml.
In general, with the stratum to be drilled becoming increasingly complicated, and with more and more deep well and ultra-deep well appeared, the temperature at the bottom hole can be up to 200-250° C. or more. Thus there exists increasing high demands for the drilling fluid technology. The fluid loss additive is one of the most critical treatment agents which can ensure the properties of the drilling fluid. The amount of the polymer fluid loss additives in the prior art has to be increased in order to play the role of reducing the filter loss in brine drilling fluid and at high temperature conditions, due to their high relative molecular weight, which causes increasing of the viscosity of the drilling fluid, or difficulties in controlling the rheological properties of the drilling fluid due to the thickening or degradation of the polymers at high temperature. Existing fluid loss additives such as sulfonated phenolic resin (SMP) can meet the demand for drilling within 200° C., but not meet the demand for filter loss control at high temperature and high pressure and demand for rheology for brine and saturated brine drilling fluid under ultra-high temperature conditions. What is needed are fluid loss additives that have all the desired properties.